1. Technical Field
The present invention relates to an image forming apparatus and image transfer apparatus including a belt-shaped image carrier that carries an image and including a transfer roller that has a partially hollow peripheral surface and that abuts on the image carrier to form a nip.
2. Related Art
In some image forming techniques by which an image is formed on a recording material such as paper, the image is temporarily formed on a belt-shaped image carrier and then is transferred to the recording material. For example, in an image forming apparatus disclosed in JP-A-2008-122815 (specifically disclosed in FIG. 1), an image forming station forms an image on a transfer belt as an image carrier that is looped around a plurality of rollers. The transfer belt and a secondary transfer roller abut on each other to form a transfer nip. A recording material is transported through the transfer nip, thereby transferring the image from the transfer belt to the recording material.
Preferably, in order for the image to be transferred from the image carrier to the recording material at a high transfer efficiency, a large pressing force is applied to the transfer material that passes the transfer nip. On the other hand, application of a large pressing force causes a disadvantage that the recording material adheres to the image carrier. For example, the image forming apparatus disclosed in JP-A-2008-122815 employs liquid development, and an electrostatic latent image is developed with a liquid carrier in which a developer is dispersed. The liquid carrier is likely to remain, resulting in the adherence of the recording material.
In order to resolve such a problem, for example, employment of a technique disclosed in JP-A-2000-508280 (specifically disclosed in FIG. 2A) is considered. An image forming apparatus disclosed in JP-A-2000-508280 is provided with an openable gripper as a gripping member at part of a peripheral surface of a cylindrical pressing roller (corresponding to a transfer roller). The gripper grips an end of a recording material, and therefore the recording material is prevented from adhering to an intermediate transfer member that abuts on the pressing roller. The intermediate transfer member, which is an image carrier in the image forming apparatus disclosed in JP-A-2000-508280, is a rotator in the form of a roller.
In terms of the combination of the techniques disclosed in JP-A-2008-122815 and JP-A-2000-508280, the following disadvantage is caused. Namely, because the pressing roller disclosed in JP-A-2000-508280 is provided with the gripping member, the peripheral surface thereof is not a smooth cylindrical surface, and therefore the pressing force at the transfer nip is varied in conjunction with a rotational period of the roller. Such a variation causes a speed variation of the belt-shaped image carrier that abuts on the roller. Consequently, an image which is formed on the image carrier by the image forming station becomes distorted.
Such a problem commonly occurs in cases where a transfer roller having a peripheral surface that is not completely cylindrical is used. For example, in cases where a transfer roller having such a shape that a cylindrical peripheral surface is partially hollow is used, such a problem may occur. Namely, the pressing force applied to a surface of the image carrier is significantly different between a case where the image carrier faces and abuts on the cylindrical peripheral surface of the transfer roller and a case where the image carrier faces the hollow portion of the transfer roller and is spaced apart from the transfer roller. Consequently, a significant speed variation is caused at the time that each of the cases is switched. Especially, such a problem significantly occurs in cases where a transfer roller including a hollow portion having a size larger than a width of the transfer nip is used.